Abstract: A system for communicating digital data over a limited bandwidth transmission link, including a transition detector for detecting transitions in the binary logic state of an input digital signal and generating a transition indicator signal in response thereto; a frequency-shift key responsive to the transition indicator signal for generating a frequency-shift keyed signal to be transmitted over the link corresponding to the logic states of the input digital signal, the frequency-shift keyed signal comprised of substantially one cycle of a high-frequency signal for a bit at one logic state and substantially one-half cycle of a low-frequency signal for a bit at the other logic state; a zero-crossing detector for detecting zero-crossings of the frequency-shift keyed signal received over the link and generating a zero-crossing indicator signal; a frequency detector responsive to the zero-crossing signal for detecting the high-frequency and low-frequency signals of the frequency shift-keyed signal, and generating

Abstract: A system for communicating digital data over a limited bandwidth transmission link, including a transition detector for detecting transitions in the binary logic state of an input digital signal and generating a transition indicator signal in response thereto; a frequency-shift key responsive to the transition indicator signal for generating a frequency-shift keyed signal to be transmitted over the link corresponding to the logic states of the input digital signal, the frequency-shift keyed signal comprised of substantially one cycle of a high-frequency signal for a bit at one logic state and substantially one-half cycle of a low-frequency signal for a bit at the other logic state; a zero-crossing detector for detecting zero-crossings of the frequency-shift keyed signal received over the link and generating a zero-crossing indicator signal; a frequency detector responsive to the zero-crossing signal for detecting the high-frequency and low-frequency signals of the frequency-shift keyed signal, and generating

Abstract: A data communication system including a central host terminal and a large number of remote terminals communicating with the host terminal through telephone lines. The remote terminal decodes frequency-shift-keyed signals and modulates an RF generator to display the data represented by the FSK signals on a conventional television screen. The remote terminal also includes a keypad and conventional two-tone signal generator for transmitting data to the host terminal. The remote terminal may also include a card reader and a frequency-shift-keyed modulator for generating FSK signals corresponding to information magnetically recorded on a card. The host terminal includes a TOUCH-TONE decoder for providing data signals indicative of the information from the remote terminals, an FSK decoder for generating data signals indicative of the information contained on cards read by the remote terminals, and an FSK modulator for transmitting FSK signals to the remote terminals.

Abstract: A data communication system in which a digital signal is applied directly to a limited bandwidth data link, such as a telephone line. Harmonies of the digital signal are attenuated either by a low-pass filter or by the limited bandwidth of the data link so that only the fundamental component of the digital signal passes through the data link. The resulting digital signal at the other end of the data link is differentiated and applied to a Schmidt trigger, which recreates the original digital signal. The data communication system may be connected to a digital device through an interface circuit which supplies appropriate signals for the serial I/O port of the digital device.